1. Field of the Invention
The present invention pertains to photosensitive devices, particularly in the form of matrices or linear arrays of photosensitive cells made out of diodes that can produce an image from an illumination. These devices may advantageously (but not exclusively) be used for the detection of radiological images.
In this field, they are exposed to a visible radiation or a near visible radiation coming from a scintillator screen. This scintillator screen has the role of converting an X-radiation that has passed through a patient to be examined or an object to be analyzed into an illumination suited to the band of wavelengths to which the photosensitive cells are sensitive.
In order to avoid unnecessary and dangerous overexposure of the patient or object to be analyzed, it is sought to interrupt the exposure as soon as the optimum dose is reached.
2. Description of the Prior Art
In standard systems of radiological imaging, there are known ways of placing ionization chambers between the patient or the object and the radiological film in order to analyze the radiation received by the radiological film. These ionization chambers cannot be used with diode-based photosensitive devices because their shadow is superimposed on the detected image and disturbs it.
In image detectors using radiological image intensifier tubes, the radiation received is controlled directly by measurement of the photocathode current. This cannot be applied to diode-based photosensitive devices.
It has been proposed, in solid state photosensitive devices, to add optical sensors to them in order perform a measurement of the illumination that they receive. These sensors give a localized estimation of a dose at a precise point but are not adapted to the morphology of the patient being examined for example, because their position is relatively permanent and their surface area is constant.
The present invention therefore proposes a photosensitive device whose cells are constituted by diodes, this device being equipped with an illumination measurement device capable of performing a measurement on predetermined cells of the photosensitive device. This measurement device directly extracts information pertaining to the illumination in real time during the illumination.
At the end of the measurement, it is then easy to interrupt the illumination by cutting off the X-ray source when the optimum dose is reached.
The place concerned by the measurement can be easily chosen as a function of the type of examination or morphology of the patient and this choice can be made before the examination but also between two image-taking operations.
More specifically, the invention pertains to a photosensitive device comprising:
photosensitive cells located at the intersection of at least one addressing conductor and one read conductor, a cell comprising a photosensitive diode connected to a read conductor and to at least one diode having a switching function connected to an addressing conductor, these diodes having a common point,
addressing means connected to at least one addressing conductor,
charge-reading means connected to at least one read conductor,
first measurement means capable of the measurement, during an illumination, of an information element representing a current flowing in at least one addressing conductor, this conductor being taken to an idle potential during the illumination so as to be capable of quantifying the illumination received by the cells located on the conductor concerned by the measurement,
and second measurement means capable of the measurement, during the illumination, of an information element representing a current flowing in at least one read conductor, this read conductor being taken, during the illumination, to a specific potential so as to be capable of quantifying the illumination received by the cells located on the conductor concerned by the measurement.
It is possible to obtain the first measurement means with at least one integrator type measurement circuit mounted between a source delivering the idle potential and the addressing conductor.
More specifically, these first measurement means may be inserted between the source and the addressing means.
These second measurement means may comprise at least one integrator type measurement circuit connected, firstly, to a source delivering the specific potential and, secondly, to at least one read conductor and at least one switch stage inserted between the source and the read conductor.
Generally, the photosensitive source is managed by a management device that controls its different operating phases. This management device may possibly also control the first measurement means and/or the second measurement means.
In this case, the information measured by the first measurement means is transmitted to the management device by means of an analog-digital converter. Similarly, the information measured by the second measurement device is transmitted to the management device by means of an analog-digital converter.
The number of addressing conductors and/or read conductors concerned by the measurement may be computed by the management device at the end of a pre-positioning illumination operation.
The location of the conductors concerned by the measurements may be computed by the management device following a pre-positioning illumination operation.
In the same way, the number of addressing conductors and/or read conductors concerned by the measurements can be determined by the operator.
It can also be envisaged that the location of the conductors concerned by the measurements will be determined beforehand by the operator.
Generally, the addressing means comprise several addressing circuits, each connected to one or more addressing conductors and it is advantageous and simple that the first measurement means should comprise as many measurement circuits as addressing circuits. Similarly, it is possible to provide that the second measurement means will comprise as many measurement circuits as integration circuits.
According to one variant, a photosensitive cell may be located at the intersection of a read conductor and a pair of addressing conductors and comprise two switching diodes series-connected in one and the same direction of conduction, each between the common point and one of the addressing conductors of the pair. In this configuration, the first measurement meads are capable, during the illumination, of measuring the information representing the current flowing in the first addressing conductor of the pair which is held at a first idle potential and the information representing the current flowing in the second addressing conductor of the pair which is held at a second idle potential, the first and second idle potentials being distinct.
Preferably, the first idle potential is negative or zero and the second idle potential is positive.
In this configuration, the first measurement means may comprise at least one pair of measurement circuits, one being mounted between a source delivering the first idle potential and the first addressing conductor of at least one pair while the other is mounted between a source delivering the second idle potential and the second addressing conductor of the pair.
The first measurement means may be inserted between the sources delivering the first and second idle potentials and the addressing means.
In this configuration, there is provision for means to add up the information from the first measurement means with the information from the second measurement means.
The management device can carry out this addition.